Put a rack in middle of oven and preheat oven to 450 degrees F. Line a baking sheet with parchment.,Whisk together mustard, vinegar, salt, and 1/4 teaspoon pepper in a large bowl.,Add chicken, tossing to coat well.,Pulse English muffins in a food processor until finely ground.,Add cheese, butter and remaining 1/4 teaspoon pepper and pulse until well combined.,Transfer to a shallow dish or pie plate.,______,Bake until golden brown, and cooked through, 15 to 20 minutes.